Filenames of some lectures are not useful, generate filename from title instead?

Some lectures don't have useful filenames for their videos (`presenter_1f840e9c_a1db_49b1_b1bb_043196952a1c.mp4` etc...). Therefore the current way of taking the file name from the download link is not perfect for usabilty.

Potential Solutions:
- generate a file name from the episode title

Potential Problems:
- special characters like `/` in episode titles need to be replaced

Start by locating the code that derives downloaded video filenames from the download link, then trace where the episode title is available. Use the example UUID-like filename and titles containing `/` to define the expected sanitized output; done means downloads use usable title-based filenames without invalid special characters.

Written by the indexing model from the issue text.